Records of cardiac arrhythmias detected in patients with implanted cardioverter defibrillators will be linked with measurements of the concentration and constituents of ambient particulate air pollution. Time series methods will be used to assess temporal associations, adjusting for seasonal, weekly, and diurnal patterns, meteorology, and co-pollutants. Characteristics of subjects showing the strongest air pollution associations will be assessed, as well as effect modification by medication and co-morbidities.